Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/es-abstract/2025/ToZeroPaddedDecimalString.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $String = GetIntrinsic('%String%');
+var $RangeError = require('es-errors/range');
+var isInteger = require('math-intrinsics/isInteger');
+
+var StringPad = require('./StringPad');
+
+// https://262.ecma-international.org/13.0/#sec-tozeropaddeddecimalstring
+
+module.exports = function ToZeroPaddedDecimalString(n, minLength) {
+	if (!isInteger(n) || n < 0) {
+		throw new $RangeError('Assertion failed: `q` must be a non-negative integer');
+	}
+	var S = $String(n);
+	return StringPad(S, minLength, '0', 'start');
+};